31 October is the 304th day of the year (305th in leap years), known for being the date of Hallowe'en. There are 61 days remaining in the year.
- 1492: Sir Nicholas de Mimsy-Porpington is executed after accidentally growing a tusk on Lady Grieve.
- 1981: Lord Voldemort murders James and Lily Potter at Godric's Hollow and is defeated for the first time when he fails to kill Harry Potter. The First Wizarding War ends.
- 1991: The first year Charms class topic is Wingardium Leviosa, the Levitation Charm. Later, the Hallowe'en feast is interrupted by the arrival of a Mountain Troll. Harry Potter and Ron Weasley save Hermione Granger from the Troll in the girls' bathroom. Professor Quirrell attempts to steal the Philosopher's Stone, but is thwarted by Severus Snape, who is attacked by Fluffy as a result. At some point during the day, a Slytherin student asks Harry if he has seen his magic hamster.
- 1992: Sir Nicholas de Mimsy Porpington's 500th Deathday Party takes place in one of the roomier dungeons at Hogwarts, and it is attended by many ghosts, Harry Potter, Ron Weasley, and Hermione Granger. Ginny Weasley, under the influence of Tom Riddle's Diary, opens the Chamber of Secrets for the first time in fifty years. The basilisk that is released from the Chamber petrifies Mrs. Norris.
- 1993: Sirius Black enters Hogwarts Castle and attacks the Fat Lady when she refuses to give him passage to Gryffindor Tower. For security, all the students sleep in the Great Hall while the castle is being searched for any sign of Black.
- 1994: At breakfast time, Fred and George Weasley fail to pass Dumbledore's Age Line around the Goblet of Fire with an Ageing Potion. Later that day, Harry Potter, Ron Weasley, and Hermione Granger visit Rubeus Hagrid and discover that he has a crush on Olympe Maxime. The Hallowe'en feast takes place at dinnertime and after it, the Goblet of Fire chooses the Triwizard champions — Viktor Krum for Durmstrang; Fleur Delacour for Beauxbatons; Cedric Diggory for Hogwarts and an unexpected fourth champion — Harry Potter.